Pseudomonas aeruginosa is known to be naturally resistant against most of the antibiotics in general use. Despite the fact that some few antibiotics are effective, the use of antibiotics is not effective in such cases as neonates who are physiologically immature in immune response or in cases when immunity depressant is administered. On the other hand, both adult human beings and animals possessing normal immunity could hardly be victims of natural infection by this bacterial genus, and even when infected, antibiotics are effective for them.
In view of the above, it can well be realized that immunity against this bacterial genus is effective for infection caused by the same bacillus. In fact, for a certain type of infection by this bacterial genus, either active or passive immunity (serum or plasma) is effective.
The thermostabile O antigen, a lipopolysaccharide (LPS) derived from Pseudomonas aeruginosa is known as an antigen originating in Pseudomonas aeruginosa and possessing the capability of protecting human, mouse and rabbit from infection caused by the homologous serotype strains of Pseudomonas aeruginosa.
However, because this O antigen is immunologically specific in its protective activity, it has the disadvantage that humans and animals immunized by a particular O antigen can be protected from infections caused only by the bacterial strains belonging to the same homologous serotype strain as the immunogen but it can hardly be protective against such infections as are due to bacterial strains whose serological specificity is heterologous.
Nevertheless, according to a method of isolating OEP from Pseudomonas aeruginosa disclosed in the British Pat. No. 1,365,950, and the product referred to therein as CWP in the above U.K. patent being equivalent to the OEP referred to in connection with the present invention, OEP almost completely free of O antigen can be isolated from Pseudomonas aeruginosa, and this OEP has been proved to be an antigen which can also protect humans, mice and rabbits from infection by different serotype strains of Pseudomonas aeruginosa to a great extent.